The term "monopulse" refers, in general, to radar tracking techniques which derive target angle-error information on the basis of a single pulse. An amplitude-comparison monopulse tracking system utilizes radio frequency (RF) signals derived from two radar beams, A and B, which are "squinted" or off-set about the axis of symmetry of a pair of radar antenna feeds. Thus, a target which is not on the axis of symmetry, will receive pulsed or continuous wave (C-W) RF energy signals from the radar antenna feeds of different amplitudes, depending upon the off-axis location of the target. This phenomena is used to develop an error signal which can be used to track the target.
The return signal is received simultaneously at both antenna feeds and a sum or reference signal (A+B) and a difference signal (A-B) is produced at the output arms of a hybrid junction device. The difference signal (A-B), when compared to the reference signal (A+B), provides an angular error signal proportional to the off-axis direction of the target.
In a conventional amplitude-comparison monopulse system, a cluster of four antenna feeds or horns, divide a primary beam into 4 subapertures which produce two pairs of beams, one pair of which is squinted horizontally and the other, vertically, to produce two-dimensional angular error signals. (See Introduction to Radar Systems by M. I. Skolnik, 1962, pp 175-184 and Phase-Amplitude Monopulse System, W. Hausz et al., 1962, IEEE, for a more complete description.) The four antenna horns are disposed about a central axis and form four quadrants; two above the horizontal axis and two below.
Phase comparison monopulse operates on a similar principle except that the orthogonal beams are produced by subdividing a primary beam at four subapertures to produce two beams in each orthogonal plane having identical amplitude versus angle response. Thus, the amplitude of the return signal from an off-axis target will be the same at each subaperture but the time of arrival will be different, producing phase difference signals (A-B) and (C-D), which can be used for tracking purposes when compared to reference signals (A+B) and (C+D).
Such conventional monopulse systems typically operate in a cartesian coordinate system in which an elevation error signal is generated from a determination of the position of the target off the Y-axis and the azimuth error signal is generated by a determination of the position of the target off the X-axis. In such systems, the monopulse antenna is driven in elevation by an elevation gimble and in azimuth by an azimuth gimble. To determine the elevation gimble error signal, the sum and difference of the return signal from the two quadrants above and the two quadrants below the horizontal or X-axis is needed. Similarly, to determine the azimuth gimble signal, the sum and difference of the two quadrants on either side of the vertical or Y-axis is needed.
Conventional cartesian coordinate monopulse systems, as described above, require a fairly complex antenna feed network of four antenna subapertures combined with four hybrids to produce the required four sum and difference signals. Furthermore, the four subapertures are located symmetrically at and about the axis of symmetry preventing location of an additional sensor, such as an IR detector, at this location, or use of a weapon system boresighted with the axis of symmetry.
Accordingly, a need exists for a less complex monopulse antenna structure and one which will permit location of additional sensors or weapons at the beam axis of symmetry or collimated to the beam axis of symmetry.